Soluble immune checkpoint molecules in patients with antineutrophil cytoplasmic antibody-associated vasculitis

Immune checkpoint molecules balance immune effector responses with regulatory reactions. We speculated that soluble immune checkpoint molecules are involved in dysregulation of the immune response and autoimmunity. We evaluated the association between soluble immune checkpoint molecules and antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). A total of 56 patients with AAV from a prospective observational cohort and 40 healthy controls (HCs) were analyzed. Soluble PD-1, PD-L1, PD-L2, CTLA-4, CD28, CD80, CD86, ICOS, TIM-3, BTLA, CD40, LAG-3, TLR-2, and CD27 were measured in stored sera using the Milliplex MAP assay. Paired analyses were performed before and after the treatment. AAV-specific indices, including Birmingham vasculitis activity score, five factor score , vasculitis damage index, and blood samples, were collected. Patients with AAV had higher levels of sPD-L1, sCD28, sCD80, sCD86, sICOS, sTIM-3, sLAG-3, sTLR-2, and sCD27 and lower level of sCTLA-4 than HCs (p < 0.05). Patients with AAV had higher serum sCD28, sCD80, sTIM-3, and sCD27 levels than HCs at baseline and decreased after treatment. Furthermore, the serum levels of sCD28 and sTIM-3 were significantly correlated with disease activity. This study demonstrated altered concentrations of serum soluble immune checkpoint molecules in patients with AAV. In particular, sCD28 and sTIM-3 may act as surrogate markers of AAV disease activity.

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a systemic vasculitis involving small vessels and includes microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic GPA (EGPA). AAV is a systemic autoimmune disease, and the presence of ANCA is a hallmark of the disease. Binding ANCA to neutrophil myeloperoxidase (MPO) or proteinase 3 (PR3) results in neutrophil activation, which causes endothelial injury and inflammation 1 .
Immune checkpoint molecules are co-stimulatory or co-inhibitory receptors and ligands that balance immune effectors and regulatory reactions 2 . Autoimmunity is prevented by immune tolerance, and immune checkpoint molecules play a decisive role in maintaining homeostasis between immune stimulation and inhibition 3,4 . Dysregulation of immune homeostasis may lead to a break of immune tolerance and develop autoimmunity 5 .
Immune checkpoint molecules are expressed on the cell surface and function by interacting with their ligands. Soluble forms of these immune checkpoint molecules are produced by proteolytic cleavage of the surface molecule or alternative splicing of mRNA 6,7 . Previous studies have shown aberrant expression of soluble checkpoint molecules in various autoimmune diseases, such as systemic lupus erythematosus (SLE) 8 , rheumatoid arthritis (RA) [9][10][11] , and autoimmune thyroid diseases 12 .
Based on these findings, we hypothesized that soluble immune checkpoint molecules are involved in immune dysregulation by interfering with the signaling between surface immune checkpoint molecules. However, little is known about the functions of soluble immune checkpoint molecules in AAV and their correlation with AAV disease activity. Here, as an attempt to further elucidate the immunopathological roles of soluble immune checkpoint molecules and to investigate for potential surrogate markers in AAV, we analysed the serum concentration and ex vivo production of various soluble immune checkpoint molecules from peripheral blood monomuclear cells Blood collection and storage. Paired blood samples were collected from consecutive patients with active and inactive AAV in the same patient. Whole blood was collected from the patients, PBMCs were isolated by Ficoll density-gradient centrifugation, and sera were immediately stored at -80 °C. Active AAV was defined as the highest BVAS state during the follow-up period, and inactive AAV was defined as the lowest BVAS state after treatment with active AAV. Of the patients with active AAV, 41 (73.2%) were sampled for newly diagnosed disease and 15 (26.8%) for relapsed disease. All patients in the inactive state of AAV were available for the second sample.
HCs. Serum levels of soluble immune checkpoint molecules were measured in the sera of 40 HCs who underwent medical check-ups. The use of clinical data from HCs was approved by the IRB of Severance Hospital (4-2017-0761).
Statistical analyses. All statistical analyses were performed using SPSS software ver. 26 (IBM Corp., Armonk, NY, USA). Categorical data are expressed as absolute and relative frequencies, and continuous data are presented as median (quartile range). The Mann-Whitney U test was performed to compare continuous variables, whereas the chi-square or Fisher's exact test was used to analyze categorical variables. The correlation coefficient was obtained using Pearson's correlation analysis or Spearman's correlation analysis for non-normally distributed data. The Wilcoxon signed-rank test was used to analyze paired data. Statistical significance was set at p < 0.05.
Ethics. This study was approved by the Institutional Review Board (IRB) of Severance Hospital (4-2016-0901) and was conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from the patients at the time of enrolment in the SHAVE cohort and blood sampling.

Serum concentrations of the soluble immune checkpoint molecules in patients with AAV and
HCs. Patients with AAV had significantly higher serum concentrations of sPD-L1, sCD28, sCD80, sICOS, sTIM-3, sLAG-3, sTLR-2, and sCD27 than HCs, and the serum concentrations of sCTLA-4 were lower in patients with AAV than those in HCs (Fig. 2).
Effect of activity of AAV on the production of soluble immune checkpoint molecules. We evaluated longitudinal changes in soluble immune checkpoint molecules from the active AAV state to the inactive AAV state after treatment. The serum concentrations of sCTLA-4, sCD28, sCD80, sCD86, sTIM-3, sBTLA, and sCD27 significantly decreased in parallel with BVAS. In contrast, the sPD-L2 levels significantly increased after treatment (Fig. 2).
Expression of PD-1, CD28, and Tim-3 on CD4 + T cells. Among the soluble immune checkpoint molecules that decreased when disease activity changed from active state to inactive state, we considered that the molecules showing a positive correlation with BVAS were significant, which were sPD-L1, sCD28 and sTIM-3. Since we tested the surface expression of T-cells, we measured PD-1, which is the receptor of PD-L1.
We collected the PBMCs from 10 patients with AAV with a high BVAS (BVAS > 15) and 10 patients with AAV with a low BVAS (BVAS < 5) to compare the cell surface expression of PD-1, CD28, and Tim-3 on CD4 + T cells. The population of PD-1 expressing CD4 + T cells was significantly higher in patients with high BVAS AAV than in those with low BVAS AAV (53.9% vs. 27.0%; p < 0.001). The population of Tim-3 expressing CD4 + T cells was significantly higher in patients with high BVAS AAV than in those with low BVAS AAV (6.35% vs. 2.42%; p = 0.006). Conversly, the population of CD28 expression in CD4 + T cells was higher in patients with low BVAS than in those with high BVAS (85% vs. 95.65%; p = 0.017) (Fig. 3).

Discussion
Previous studies have demonstrated altered cell surface expression of checkpoint molecules in various autoimmune diseases [18][19][20] . Furthermore, it was considered that their aberrant expression could switch off immune tolerance, leading to the initiation of autoimmunity 4,21 . However, there has been no study clarifying the levels of the production of soluble forms of immune checkpoint molecules and determining their clinical roles in AAV patients to date. This study demonstrated that the aberrant production of soluble checkpoint molecules might be associated with AAV. To the best of our knowledge, this is the first study to elucidate the association of the soluble immune checkpoint molecules with AAV-specific indices and acute phase reactants. Our results showed that sPD-L1, sCD28, sCD80, sICOS, sTIM-3, sLAG-3, sTLR-2, and sCD27 were significantly higher and sCTLA-4 was significantly lower in patients with AAV than those in HCs. The presence and altered production of these soluble checkpoint molecules may indicate that they play an important role in the derangement of immune homeostasis.
It is noteworthy that patients with active AAV had higher concentrations of sCD28 and sTIM-3 than HCs, which decreased as the disease improved after treatment. Furthermore, these soluble molecules were positively correlated with BVAS and CRP levels, indicating that sCD28 and sTIM-3 reflect disease activity. These findings are consistent with those of previous studies, which reported that plasma sCD28 and sTIM-3 concentrations were increased in patients with SLE and correlated with disease activity [22][23][24] . Additionally, a previous study showed that sCD28 concentrations were elevated in patients with RA and decreased after treatment 9 . The above observations and our results suggest that sCD28 and sTIM-3 could potentially serve as a surrogate biomarker of AAV disease activity. Based on these findings, we expected that soluble immune checkpoint molecules would be clinically useful if they could predict disease relapse or treatment failure. Additional prospective studies with serial measurements of the soluble checkpoint molecules would provide more reliable information regarding the clinical implications of using the soluble checkpoint molecules as a disease activity indicator for AAV patients.
Soluble immune checkpoint molecules are produced by proteolytic cleavage of the surface molecule or alternative splicing of mRNA [6.7]. There may be a possibility that the membrane-bound receptors may be released to the blood circulation from cells through apoptosis or necrosis; however, since their molecular weights are relatively heavy, and thus those receptors are generally precipitated by centrifugation, this possibility could be ignored.
CD28 is proteolytically cleaved in activated T cells by matrix metalloproteinases (MMP) 2 and MMP 13 25 , resulting in increased serum sCD28 levels. In our data, sCD28 levels were higher in patients with active AAV than www.nature.com/scientificreports/ in those with inactive AAV or HCs, and the surface expression of CD28 in CD4 + T cells was lower in patients with high BVAS. We speculated that the shedding of surface CD28 in its soluble form acts as a regulatory mechanism that inhibits the CD27:B7 activation pathway. The surface expression of PD-1 and Tim-3 on CD4 + T cells was higher in patients with a high BVAS than in those with a low BVAS in our data. PD-1 and Tim-3 are representative inhibitory molecules, which can be explained by the fact that persistent stimulation of T cell receptors provoke PD-1 and Tim-3 production to regulate excessive activation. Consequently, sPD-1, which is produced by the alternative splicing of PD-1 mRNA transcripts, is increased along with enhanced PD-1 production (Fig. 4).
sTim-3 is produced by shedding from the cell surface by a disintegrin and metalloproteinase domain-containing protein (ADAM) 10 or ADAM17 in humans 26,27 . Our results showed that surface Tim-3 was overexpressed in the T cells of patients with high BVAS, which is the source of sTim-3. Consequently, sTim-3 is elevated in patients with active AAV compared with those with inactive AAV. We speculated that activated immunity enhances Tim-3 expression to maintain immune homeostasis, similar to PD-1. However, the immunopathological properties of these soluble molecules in patients with AAV require further investigation.
Not all the molecules showed significant results. For instance, serum sCTLA-4 and sBTLA levels were lower than those of HCs, but decreased further after treatment, and serum sLAG-3 level was higher than those of HCs, but increased further after treatment. Since specific mechanism of production and function of these soluble molecules are little known, further studies are needed to determine how and why these molecules changed in response to treatment.
However, we speculated that chronic immune activation status may influence dysregulation of checkpoint molecules. Dysregulation of soluble immune checkpoint molecules has also been reported in infectious disease other than autoimmune diseases such as hepatitis B virus (HBV), human immunodeficiency virus (HIV), and tuberculosis infection [28][29][30] . Although detailed function, regulation and mechanism of soluble immune checkpoint molecules in each disease are largely unknown, these findings suggest the possibility that chronic immune activation status may influence dysregulation of checkpoint molecules. www.nature.com/scientificreports/ The limitations of this study include the following: first, our data should be interpreted with caution because our results are phenomenological; thus, corroborating mechanistic data are needed to elucidate the underlying mechanism. Downstream signaling following interactions between membrane-bound receptors and soluble checkpoint molecules has not yet been clarified. Second, 15 patients relapsed and were not newly diagnosed; therefore, they were not immunosuppressant-naïve. Therefore, it is possible that the medications affected the soluble checkpoint molecules. Third, all patients were Korean; thus, further studies of various ethnicities are required to validate our results. Fourth, because Miliplex MAP testing is a relatively recently developed technology and we do not have separate validation group reliability and stability test of the Miliplex MAP should be validated in other AAV cohorts in future studies. Nevertheless, the strength of our study is that it is the first to analyze various soluble checkpoint molecules and their associations with AAV-specific indices in a prospective AAV cohort with paired samples.
In conclusion, we demonstrated altered concentrations of serum soluble checkpoint molecules in patients with AAV, as well as a positive correlation between serum sCD28 and sTim-3 concentrations and disease activity. Taken together with our novel findings on the aberrant production of various soluble checkpoint molecules, our results provide new insights into their potential immunopathological roles in AAV and offer novel biomarkers of disease activity in AAV.

Data availability
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request (sangwonleee@yuhs.ac).